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The California condor is the largest bird in North America. it is an endangered species. 20 years ago there were less than 30 birds left. All of the birds were trapped and kept in safe places. The number of condors grew larger. Puppets that looked like condors were used to take care of babies so that more eggs could be hatched.In the last few years, some of the condors have been returned to the wild. But they seem to like humans now and come into danger around people. Some birds perch on houses and tear up roofs. Some have even gone into houses. Birds have been shot, poisoned, and killed by power lines.Now there is another problem. Condors find dead animals that hunters have killed. The bullets cause lead to get into the meat and it can kill the condors. Some people think hunters should use different bullets. Other people think that the birds should not be put into the wild. If the condor population has tripled since two decades ago, about how many are there now()
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A new kind of radar has been developed for spaceship travelers. A working laboratory model of a new system of radar that makes use of a beam of light is said to be ten thousand times more accurate than the best comparable system of radar that used microwaves.The model has shown that this radar system, known as laser-doppler radar, can measure with absolute precision speeds varying from spaceship orbital injection (进入) velocities (速度) of five miles per second down to virtual stops—speeds of less than one ten thousandth of an inch per second. According to the scientists who are developing this system, such fine measures of velocity are of prime importance in space missions. In a rendezvous (对接) between two spaceships, or in a landing approach by a vehicle onto an orbiting space station, a bump could rip open a ship’s skin, or a nudge could knock the station out of its orbit.The light-beam radar, which operates at a frequency of trillions (百万兆) of cycles per second, could easily detect and measure the movement of a vehicle edging up to a satellite space station. A control system using so precise a signal as this would allow a huge vehicle to dock at a space station as lightly as a feather. Implied but not stated is that ()
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